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arm64: Disable kernel superpage promotion when KMSAN is configured

Authored by markj on Dec 21 2023, 6:40 PM.
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The break-before-make operation required to promote or demote a
superpage leaves a window where the KMSAN runtime can trigger a fatal
data abort. More specifically, the code in pmap_update_entry() which
executes after ATTR_DESCR_VALID is cleared may implicitly attempt to
access KMSAN context via curthread, but we may be promoting or demoting
a 2MB page containing the curthread structure.

Sponsored by: Klara, Inc.
Sponsored by: Juniper Networks, Inc.

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